1. Field of the Invention
The present invention relates to electronic devices, and in particular, to a cooling structure for transferring heat out of electronic components and methods of manufacturing such cooling structure.
2. Description of Related Art
Modern electronic devices often employ chips having increased power densities due to the continually increasing transistor densities, chip operating frequencies and current leakages. However, current cooling structures and capabilities are generally not scalable to meet the demands of these increased power densities.
Conventional thermal technologies for cooling electronic devices include the use of heat spreaders, heat sinks, and associated thermal interfaces. However, these conventional technologies are insufficient for cooling electronic devices when the cooling fluid thermal budget is small. Conventional cooling technologies are also insufficient for cooling electronic devices having high average power flux, and/or high local power flux (hot spots), such as those having average power flux above 200 W/cm2 and even very high average power flux exceeding 400 W/cm2. For instance, in electronic devices having such high average power flux, a high power hot spot may dissipate 300 to 500 W/cm2, while a very high power hot spot may dissipate more than 500 W/cm2. Under such processing conditions, if the cooling method is primarily based on a heat conduction mechanism, the power flux will be sufficiently large such that it generates significant thermal gradients along the cooling axis.
In these high power flux situations that generate significant thermal gradients along the cooling axis, acceptable cooling techniques typically require the use of single phase fast forced convection and/or two phase evaporation based devices with a circulating fluid. However, higher cooling solutions often require higher fluid flow rates which result in an increase in pressure drop on the cooler. Yet, known cooling solutions based on a circulating fluid have a maximum operating pressure for preventing fluid leakage and/or mechanical damage to the system. Thus, in both cases, the extendibility of known cooling solutions is deleteriously limited by the pressure drop required to operate the cooler in high power flux situations.
For instance, FIG. 1 illustrates a prior art heating/cooling device 5 that includes a cluster or array of fluid jets 11 impinging orthogonally on a surface 7 of an electronic component 9, such as a semiconductor chip, and a lateral drain 12 for removing the spent fluid. The fluid jet array 11 provides a high heat transfer rate when used to heat or cool the surface 7, as compared to conventional convection heating and/or cooling processes.
In this scenario, the high kinetic energy of the array of fluid jets 11 provides fresh jet fluid in close proximity to surface 7 in the region directly below the array of jets, generally at the center of the electronic component, thereby enabling high heating or cooling rates at such regions. However, this high heating/cooling transfer rate decreases rapidly in areas of surface 7 not residing directly below the surface area impinged by the jet fluid. This undesirably results in uneven cooling across the surface of the electronic component, particularly from the center to the edge of the electronic component.
These conventional heating/cooling devices that employ a cluster of fluid jets in combination with the lateral drain 12 are also insufficient for cooling high power flux situations due to the lack of proper spent fluid drainage. These lateral drains remove the spent fluid in a radial flow pattern away from a fluid jet at the center of the surface. In so doing, the drain velocity increases approximately linearly with the number of fluid jet rows encountered from the central jet row(s) to the fluid drain outlet located at the periphery of such jet array, thereby reaching a maximum velocity at such periphery.
These types of conventional fluid jet arrays are also impractical for cooling high power flux situations due to their structural designs. Conventional single-phase jet array structural designs limit the drain velocity such that it does not exceed 50% of the fluid jet array velocity. Drain velocities above 50% of the fluid jet array velocity undesirably force the fluid jets to move away from the desired orthogonal orientation, thereby reducing the heat transfer rate by more than 20% relative to the heat transfer rate of a fully orthogonally oriented jet. This problem increases significantly in two-phase coolers, where the gas phase volumetric flow can be up to three orders of magnitude larger than the liquid flow.
The thermal performance of prior art fluid jet arrays has been studied extensively, both theoretically and experimentally, particularly those having high jet Reynolds numbers, e.g., jet Reynolds numbers over 2000. In so doing, it has been found that high jet Reynolds numbers, along with optimization of the fluid jet array, can maximize the thermal performance of a given jet cluster geometry. However, such high jet Reynolds numbers can also undesirably result in a large pressure drop on the cooling device, thereby rendering it useless for its intended purpose.
Further, in conventional fluid jet arrays of current cooling/heating devices, an evaluation of the thermal performance thereof. Indicates that the heat transfer coefficient increases with decreasing jet diameter. However, existing optimization guidelines for current fluid jet arrays requires well-defined jet pitch and gap height (i.e., spacing between the jet array and the target surface) to maximize heat transfer rates. This, often requires fluid jet diameters as small as feasible, with a maximum jet velocity limited by keeping the cooler total pressure drop below a given practical limit, for example 10 psig, and operating as close to the surface as required by the geometric optimization algorithm.
Unfortunately, these parameters critically limit conventional fluid jets to have fewer than approximately 200 individual jets if a relatively uniform and high heat transfer rate is desired. For example, to maximize the cooling capability of a high power silicon chip with a hot surface area of 400 mm2 using a conventional cooling device, the cooling device should have 196 fluid jets with a minimum jet pitch of about 1400 microns, jet diameter of 400 microns or more, and minimum gap height of 1600 microns. Any deviations to smaller dimensions will undesirably result in lower thermal performance, thereby resulting in insufficient cooling of the chip. Further, using these conventional devices, cooling is undesirably varied across the surface of the chip. For instance, if the coolant is water with an inlet temperature of 22° C. and average jet velocity of 2 m/s, and the chip has a thickness of about 0.75 mm with a maximum junction temperature at 85° C., then the maximum average heat flux capability of the cooling/heating device at the chip center is approximately 170 W/cm2, while this capability will get reduced to less than 135 W/cm2 at the chip edge.
Accordingly, a need exists for improved methods of maximizing the heat transfer rate of fluid jet arrays for cooling, or heating, components having high power flux, which generates significant thermal gradients along the targeted surface.